1. Field of the Invention
The present invention relates to a scanning unit for used in an image-forming device, and more particularly to a scanning unit including a polygon mirror for scanning light beams, the polygon mirror having a plurality of reflecting surfaces on which a plurality of light beams are obliquely incident from different positions in a subscanning direction.
2. Description of the Related Art
In a laser printer or other electrophotographic image-forming device, a light source such as a semiconductor laser emits a light beam that is deflected by a polygon mirror or other deflecting means onto a photosensitive member, the surface of which has a uniform charge, so as to form an electrostatic latent image on the surface of the photosensitive member by scanning the light beam thereon. The latent image is subsequently developed into a visible image with toner, and the toner image is transferred onto a recording medium, such as a sheet of paper, to form an image on the recording medium.
Conventional image-forming devices form color images by superimposing toner images in the colors cyan (C), magenta (M), yellow (Y), and black (K). Color image-forming devices that employ the tandem system are provided with a plurality of image-carrying members (photosensitive drums or the like), one for each color. The surface of each image-carrying member is exposed by a light beam according to image data corresponding to each color image, forming a latent image that is subsequently developed with toner. The toner images of each color are then transferred sequentially and superimposed over one another on an intermediate transfer medium or a paper or other recording medium, thereby forming a color image.
When the image-forming device includes a plurality of image-carrying members as in the tandem system (or scans a single image-carrying member at different exposure positions) to form color images by sequentially superimposing toner images in each color, problems in color registration can occur when the shape of the scan lines for each color do not match, resulting in a loss of image quality. Color registration is particularly difficult when light beams are irradiated so as to be obliquely incident on the reflecting surfaces of the polygon mirror, because the distance that the light beam travels from the reflecting surface to the fθ lens is different than when the light beam is not obliquely incident and produces a curved scan line.
Japanese Patent Application Publication No. 10-221617 describes an optical scanning device employing polygon mirrors to deflect a plurality of light beams emitted from light sources corresponding to each color. In this technology, the optical scanning device employs two polygon mirrors, and light beams are emitted so as to be obliquely incident on the front surface and rear surface sides of each polygon mirror. Optical reflecting systems corresponding to each of the plurality of light beams reflected off the reflecting surfaces of the polygon mirrors are configured of reflecting mirrors or other reflective optical reflecting elements for guiding the respective light beams to the corresponding image-carrying members in order to expose the surfaces of the image-carrying members.
In the technology described in Japanese Patent Application Publication No. 10-221617, the number of reflective optical reflecting elements constituting each optical reflecting system is adjusted so that all light beams expose the photosensitive members from the same side in order to produce uniform scan line curves.
However, there has been much demand in recent years to produce more compact image-forming devices, such as the tandem type color image-forming devices described above for forming color images using a plurality of image-carrying members. Here, the use of two polygon mirrors as described in Japanese Patent Application Publication No. 10-221617 does not contribute to the production of a more compact device